This invention relates to motherboard add-in cards and, more particularly, to an add-in card used in high-frequency signaling operations.
Printed circuit boards (PCBs) are single or multiple-layered platforms upon or within which printed conductors and electrical components are mounted. The components are connected together by the printed conductors that are either embedded within layers of the PCB or etched upon its surface. In addition to providing support, the PCB is a thermal conductive path for heat dissipation and an insulator for the conductors.
The PCB base or substrate is made from an insulator such as phenolic or fiberglass material. Electrical signals pass through the components along the conductors, which are made from copper or other highly conductive material. Some components are permanently affixed to the PCB, while others are connected using edge connectors, cartridges, and other support assemblies.
A conventional add-in card is a PCB that inserts into a connector on a receiving PCB, such as a motherboard of a personal computer. A terminating edge of the add-in card is inserted into the connector. One or both sides of the terminating edge include several closely spaced conductive fingers, known as edge fingers, extending transversely along its length. Formed along with the conductors and component mounting pads, the edge fingers may be gold-plated or otherwise coated with a non-corrosive material, such that insertion and removal does not damage the edge fingers. The edge fingers are spaced apart and typically are raised from their respective outer layer slightly.
The connector likewise includes plated surfaces against which the edge fingers make contact when the add-in card is inserted. The plated surfaces may flex to allow the edge fingers to frictionally engage with the connector. The connector may be permanent or non-permanent, such as to engage a removable add-in card. The terminating edge of the add-in card may be beveled to facilitate insertion into the connector. Like the motherboard, the add-in card may be single-sided, double-sided, or multi-layered.
The PCB and the support assemblies are designed to ensure that the electrical signals pass from the PCB to the add-on component, and back, without interruption of the signal transmission. Each component has an electrical characteristic, known as impedance, with which signal loss can be predicted. Impedance is the opposition to the flow of electrical current by the component, expressed in Ohms. Ideally, the PCB and its components have impedance characteristics that are identical to the connector.
Impedance mismatch can occur anywhere along a signal path, such as when the signal transitions between PCBs. Thus, for an add-in card connected to a motherboard, an impedance mismatch can occur when the signal passes from the motherboard to the connector, and again when the signal passes from the connector to the add-in card.
Insertion loss is the attenuation of signals as they pass through the connector. For a typical 100 Ohm differential bus (50 Ohm single-ended), impedance mismatch causes insertion loss, which is a function of frequency, and which becomes more pronounced at higher frequencies. High-performance connectors have been developed to ensure that signals reaching add-in cards at these speeds avoid failures due to high insertion loss. It is well known, however, that the performance of the connector alone does not prevent such failures. Instead, how the signal path transitions between the motherboard and the add-in card greatly impacts the connector performance.
In a multi-layered PCB, one or more layers, known collectively as reference layers, can be dedicated to power (power plane) or to ground (ground plane). Conventionally, the ground and power planes extend entirely throughout the add-in card, including the terminating edge. Sometimes, the ground/power planes adjacent to the edge fingers cause too much capacitance at the edge fingers, resulting in a severe impedance mismatch between the add-in card and the connector. The impedance mismatch can cause the signal to reflect back, resulting in signal distortion and possibly even data loss.
One solution is to completely remove the ground and power planes underneath the edge fingers. This can remove too much capacitance, however, and the impedance mismatch remains. Further, cross-talk between the signals may result. Cross-talk occurs when a signal from one input (or channel) crosses with a signal from another input (or channel). Cross-talk is particularly problematic in systems with high insertion loss, where those systems employ differential signaling.
Differential signals, or differential pairs, are groups of signals that are routed together and are approximately the same length. The signal value on each path is the difference between the individual voltages on each path (in contrast to measuring the voltage against a ground voltage). Differential signaling may be preferred where the signal source is far from the signal receiver or where electromagnetic interference (EMI) is a problem, such as where high-frequency signals are transmitted.
Add-in cards that transmit differential signals are particularly vulnerable to cross-talk, complicating the insertion loss problem. This is especially so when high-speed signals are transmitted through the add-in card. Thus, there is a continuing need to design an add-in card that minimizes insertion loss and cross-talk, particularly when high frequency signals are passing through the connector.